1. Field of the Invention
The present invention relates to a liquid ejection head having orifices for ejecting liquid and an image-forming device using the same.
In this specification, the word “print” refers to not only forming significant information, such as characters and figures, but also forming images, designs or patterns on a printing medium and processing, such as etching and so forth, of the printing medium, whether the information is significant or insignificant and whether it is visible so as to be perceived by humans or not. The term “printing medium” includes not only paper used in common printing apparatus, but also sheet materials such as cloths, plastic films, metal sheets, glass plates, ceramic sheets, wood panels and leathers, and three-dimensional materials such as spheres, round pipes and so forth which can receive the ink. The word “ink” should be interpreted in its wide sense as with the word “print”, and it refers to liquid that is applied to the printing medium for forming images, designs or patterns, processing, such as etching, of the printing medium, or processing, such as coagulating or insolubilizing a colorant in the ink, and it includes any liquids used for printing.
2. Description of the Related Art
Recently, demand for high gradation color printing has risen as the internet or digital cameras become popular, and ink jet printers having a higher performance have been developed therewith. The following methods (1) to (3) are known for obtaining a high precision, high gradation and high quality printed image:                (1) The arrangement pitch of orifices for ejecting ink is minimized to improve the resolution.        (2) A plurality of print heads, each ejecting (at least two kinds of) a specific color ink containing a coloring material of different ratios, i.e., different color concentrations, are prepared, and a deep ink and a light ink are selectively printed, one over the other if necessary, so that the gradation is improved.        (3) By varying a size or an amount of an ink droplet ejected from the orifice, the gradation is improved.        
Since the above-mentioned method (3) is relatively difficult to be performed in a so-called bubble-jet type printer in which thermal energy is used for generating a bubble in the ink, the blowing pressure of which is used as energy for ejecting ink from the orifice of the print head, it is thought that the methods (1) and (2) are particularly effective for the bubble-jet type printer.
To realize the method (2), however, two or more print heads are necessary for a specific color ink, which results in a high cost. Accordingly, for the bubble-jet type printer, it is most preferable and convenient to adopt a method in which the arrangement pitch of the ejection openings is reduced as in the method (1) and the size of an individual ink droplet ejected from the respective ejection opening is minimized (for example, to 10 pico-liters or less) so that the resolution is improved. This is because the production cost hardly rises by this method.
A type of printer that communicates a bubble to the atmosphere via an ejection opening when a small ink droplet is ejected from the ejection opening, in which the bubble grows with the heating of ink due to the film boiling, is disclosed, for example, in Japanese Patent Application Laid-open Nos. 4-10940 (1992), 4-10941 (1992) and 4-10942 (1992). To differentiate such a type from the conventional bubble-jet type in which the ink droplet is ejected without communicating the bubble, which is growing due to the film boiling, with the atmosphere, the former type may be called a bubble-through type.
In the print head of the conventional bubble-jet type in which the ink droplet is ejected without communicating the bubble growing due to the film boiling with the atmosphere, it is necessary to reduce the cross-sectional area of the ink passage communicating with the ejection opening as the size of the ink droplet ejected from the ejection opening becomes smaller. Thereby, an inconvenience may occur in that the ejection speed of the ink droplet is decelerated because of the lowering of ejection efficiency. If the ejection speed of the ink droplet decelerates, the ejecting direction becomes unstable. In addition, the ink becomes gradually viscous, as moisture is vaporized while the print head is inoperative, causing the ink-ejection to be more unstable, resulting in premature ejection failure or other problems. As a result, the reliability may be lowered.
In this respect, the bubble-through type print head in which a bubble communicates with the atmosphere is suitable for ejecting an ink droplet, since the size of the ink droplet can be decided solely by the geometric configuration of the ejection opening. In addition, the bubble-through type print head is advantageous in that it is hardly affected by temperature or other factors and the ejection rate of the ink droplet is very stable in comparison with the conventional bubble-jet type print head. Accordingly, it is possible to relatively easily obtain a high precision, high gradation and high quality printed image.
To obtain the high precision, high gradation and high quality printed image, preferably, an ink droplet containing an extremely small amount of ink is ejected from an individual ejection opening during the printing operation. In this case, it is necessary to eject ink droplets from the ejection opening in a short period for the purpose of obtaining a high printing speed. Further, it is necessary to make the carriage carrying the print head thereon scan at a high speed relative to a printing medium in synchronism with a drive frequency of the print head. From this point of view, it could be said that the bubble-through type is particularly suitable for the ink jet printer.
A state of the ejection of ink droplet is depicted in FIG. 11, when a so-called “solid” printing is carried out on a printing medium, in which ink droplets are continuously ejected from all the ejection openings while subjecting the print head of such an ink jet type to scanning movement at a high speed together with the carriage along the printing medium. The direction of the scanning movement of the print head 1 is perpendicular to the plane of the paper in FIG. 11, and the non-illustrated ejection openings are arranged leftward and rightward in the drawing. When the image data is “solid”, all of the ejection energy generating means (not shown) corresponding to the respective ejection openings are driven at a high driving frequency. Therefore, viscous air around the ink droplet 3 ejected from the ejection opening toward the printing medium 2 is also entrained therewith. As a result, a surface area 4 of the print head 1 containing the ejection openings of the print head is more decompressed than the periphery of the print head 1. Particularly, it has been found that the ink droplets 3 ejected from the ejection openings located at opposite ends of the orifice arrangement are sucked toward a center of the arrangement, whereby those ink droplets are not directed to their intended predetermined positions on the printing medium 2.
In addition, as is apparent from the graph of FIG. 12 illustrating the relationship between the total number of ejection openings actually used and the amount of positional deflection of an ink droplet ejected from an ejection opening located at the arrangement end relative to the printing medium, the deflection of the ejecting direction of the ink droplet 3 by the influence of the above-mentioned air stream becomes significant generally in proportional to the total number of ejection openings actually used.
FIG. 13 schematically illustrates a solid printed image formed on the printing medium when the scanning movement of the carriage is repeated under such a phenomenon. The carriage moves together with the print head from an upper area to a lower area in the drawing. It will be understood that, in this case, a white streak 7 is formed between a solid image 5 formed by the preceding scanning movement and another solid image 6 formed by the subsequent scanning movement.
Such an inconvenience is particularly significant in the bubble-through type ink jet printer having a small arrangement pitch of the ejection openings and capable of ejecting ink droplets containing a small amount of ink, as little as 10 pico-liters or less, in a short period by one drive operation.
To avoid this inconvenience, it is also possible to restrict the deflection of ejection of the ink droplets ejected from the ejection openings located at the respective opposite arrangement ends by enlarging the size of the ink droplets, i.e., by increasing the inertia mass of the ink droplets ejected from the ejection openings of the respective opposite arrangement ends. The enlargement of the ink droplet size, however, hinders the formation of a high precision and high gradation image. Further, the permeation of the ink droplets into the printing medium is retarded, and the printed image is liable to deteriorate with the swell of the printing medium. Alternatively, it is also possible to mitigate the above-mentioned inconvenience by suppressing the drive frequency for the ejection energy generating means to a lower level. When the drive frequency for the ejection energy generating means is set to a lower level, however, the printing speed becomes too slow to satisfy the user's need for obtaining high speed printing.